As communications processors and wireless mobile devices incorporating such processors become increasingly complex, power consumption may increase. Sleep modes of operation and base station paging protocols have been developed to conserve battery power. Transmissions between the mobile device and a base station may be suspended during a period of sleep-mode operation. The mobile device may wake up every few seconds according to the paging protocols to engage in active communication with the base station. Data or incoming calls may be transferred to the mobile device during these wakeful periods.
Accurate time interval measurements may be required while a wireless mobile device is in sleep mode. The wireless mobile device may require at least one time base in order to perform the time interval measurements and to establish communication according to the paging protocols. The time base may comprise a crystal oscillator-based clock local to the device. The clock may be locked onto a remote time base associated with the base station during periods of active communication. In order to maintain synchronization with the base station, the wake-up timing may require a particular accuracy, perhaps to a few microseconds.
Thus, a time base with an accuracy of a few parts per million (PPM) may be required to be active during sleep-mode periods. Some wireless mobile devices may use a time base other than a main crystal oscillator to maintain the sleep-mode time base. The high frequency of operation associated with the main crystal oscillator (e.g., 26 MHz in a cellular telephone) may result in an unacceptable level of power consumption during the sleep-mode period. A lower-frequency crystal oscillator capable of continuous operation with a small battery drain, including perhaps a 32.768 KHz oscillator, may be used instead. However the lower-frequency oscillator may be less accurate than the higher-frequency main oscillator.
At the start of a paging frame the wireless mobile device may be powered on, with both clocks active. At the end of the paging frame, before entering sleep mode, the device may store in memory a number of fast clock cycles counted from the beginning of the paging frame until the closest edge of the slow clock. This may enable the device, upon wake-up, to calculate a number of fast clock periods which, when added to an edge of the slow clock, cause the edge of the slow clock to occur at the beginning of a next paging frame. That is, the wireless mobile device may wake up and begin using the main oscillator some time before the start of a paging frame. Likewise, the device may continue to use the more accurate main oscillator for some period of time following the end of the paging frame, before entering sleep mode. The accuracy associated with the main oscillator may thus be blended with the accuracy associated with the lower-frequency oscillator to yield an overall accuracy during a period of time.
The lower frequency crystal oscillator may be used as both a sleep-mode system clock and as a real-time clock (RTC). Implementing the sleep-mode clock with the lower frequency crystal oscillator may incur substantial cost, however, since discrete components including the crystal may be used in these designs.